1.) General Discussion
Within the industrial heat treat art, industrial heat treat furnaces can be broadly classified as being either vacuum furnaces or atmosphere furnaces. Procedurally the manner in which heat treat processes are performed in vacuum furnaces is fundamentally different than the manner in which heat treat processes are performed in atmosphere furnaces. In atmosphere furnaces the work is immersed in a gaseous furnace atmosphere which is controlled at temperature, composition, flow rate, etc. so that gas reactions will proceed to equilibrium in a manner which permits the work to undergo whatever phase transformation or alteration is desired to achieve the specific heat treat process. In vacuum furnaces, the furnace atmosphere is removed by a vacuum pump and the work heated to the desired temperature to effect the heat treat process. Both furnaces then use similar mechanisms to quench or cool the work. Both furnaces purge the furnace chamber during preheat of the work. The furnaces differ during that portion of the heat treat process where the work is heated to its heat treating temperature.
Although it is difficult to make any general statements in this area, almost all heat treat processes can be performed in a vacuum furnace, although not all heat treat processes can be performed in atmosphere furnaces and for certain heat treat processes, vacuum furnaces inherently produce better or more closely controlled heat treated product. On the other hand economics dictate use of atmosphere furnaces in terms of capital cost, throughput, capacity, operating costs etc. For both furnace types, continuing efforts are being made to more closely control the heat treat process to improve product quality.
Vacuum furnaces are typically constructed with a double wall steel casing containing a water jacket therebetween in a pressure vessel configuration with doors sealed by water cooled elastomer seals so that a fine or hard vacuum can be drawn within the vessel. Because of sealing requirements, cold wall vacuum furnaces are universally electrically heated by graphite heating elements which traditionally increase energy costs when compared to gas fired burners. In contrast, atmosphere furnaces are typically constructed with a single 1/8"-1/4" thick steel casing to which is attached blanket insulation i.e. a hot wall design. As explained in the chapter entitled "Vacuum Furnace" set forth in Volume 4, "Heat Treating" of Metals Handbook, Ninth Edition, there are hot wall vacuum furnaces. These furnaces employ a retort, usually cylindrical in configuration, through which a vacuum is pulled and the retort is inserted into a refractory lined atmosphere type furnace. However, standard construction techniques for today's atmosphere type furnaces produce a sufficiently "tight" furnace to permit a soft or slight vacuum to be drawn or pulled in such furnace, and this is the furnace to which the present invention relates. The furnace to which this invention relates can be classified as a "hot wall" vacuum furnace but is differentiated from conventional hot wall vacuum furnaces in that no retort is used.
Traditionally gas fired atmosphere furnaces have been limited to processing temperatures of 1850.degree. to 1900.degree. F. although recent integral-quench technology developed by Surface Combustion, Inc. under funding from Gas Research Institute has extended that capability to temperatures up to about 2050.degree. F. and is practiced in Surface's UltraCase furnace. There is however, an increasing range of materials that require processing above 2050.degree. F. This range includes tool and die steels, heat resistant alloys, powder metals and a wide range of structural ceramics and composites. Such materials which require processing at temperature ranges of from 2050.degree. F. to 2350.degree. F. are currently processed, until the present invention, in vacuum furnaces. The only other viable alternative is heat treating in molten salt baths which is gradually becoming cost prohibitive because of environmental concerns.
With respect to the control systems now in use for vacuum furnaces for non-case hardening processes, because of the hard or fine vacuum, the oxygen content of whatever residual gas remains in the furnace, if converted to water vapor, would produce lower dew points then that achieved in atmosphere furnaces with inert or reducing gas atmospheres. Thus as a general concept, control of vacuum furnaces during the heating stage is simply one of time and temperature established in advance at vacuum levels which will not produce a vapor pressure which will draw alloying elements out of the workpiece. Of course prior to heating, the vacuum chamber is purged with an inert gas and after heating, cooling is effected by conventional practices common to both atmosphere and vacuum furnaces. While this is the general control scheme, heat treating is an advanced art and there are vacuum furnace installations for certain heat treat processes where the vacuum is reduced to avoid vaporization. This has been accomplished by means of a gas such as nitrogen which is injected in small quantities into the vacuum vessel by a needle valve. This deliberate leaking of gas raises the pressure within the vacuum furnace to some stable level between the ultimate vacuum of the pumping system and atmospheric pressure. For example, the vaporization of copper during the brazing of heat exchangers is suppressed by the leaking of nitrogen into the furnace and maintaining the vessel at a vacuum level of 0.27 kPa (2.0 torr). This is typically an intermittent operation.
In performing case hardening heat treat processes with vacuum furnaces, typically carburizing, a carbon bearing gas such as methane, is introduced into the furnace while the work is heated in a vacuum. Typically, a fixed quantity of methane is metered into the furnace over a pre-established time period, and then stopped while the work is maintained at the carburizing temperature or soaked prior to being quenched. In some processes the methane content of the gas pulled from the furnace by the vacuum pump is analyzed by a Residual Gas Analyzer to determine how much carbon has been deposited on the case. There is no dew point or oxygen reading and there is no continuous sampling of furnace gas because there is no component of the gas within the furnace to which the reading can be compared. In some instances, vacuum carburizing is performed by metering methane with a carrier gas i.e. nitrogen which, after it is drawn out of the furnace, is enriched with methane and recycled back into the furnace in a closed loop system. The content of the spent gas may be analyzed prior to enriching with "makeup" methane by various gas sampling techniques which would compare whatever was sensed in the vacuum stream with an ideal methane-inert gas composition to add the desired methane. Outside of these "exceptions", the general control scheme for a vacuum furnace is simply time, temperature and vacuum levels. In addition, while the work is being heated i.e. preheat, conventional practice is to purge the vessel with a backfill of an inert gas such as nitrogen or argon. Normal practice is to backfill the vessel with nitrogen, while the work is being heated and then pump the vessel down to a fine vacuum and repeat the backfill for about 5 to 10 purges of the furnace.
Control schemes for atmosphere furnaces are different than for vacuum furnaces. In non-case hardening processes, the general control concept is, after purging, to continuously inject, at a somewhat constant mass flow rate, a treating gas. The gas within the furnace is then analyzed by various devices such as a CO.sub.2 analyzer to determine the partial pressure of the sensed gas which reacts with other gases in the furnace to reach equilibrium. Depending upon the readings a "scavenger" gas or (gases) is then introduced into the furnace to readjust the equilibrium balance of the gases. Insofar as the present invention is concerned, it is conventional practice to use a neutral or inert gas such as nitrogen or argon to which an enriching gas such as hydrogen is added to produce an enriching atmosphere within the furnace for certain heat treat processes. Dry hydrogen atmospheres are used in the annealing of stainless and low carbon steels, electrical steels, several non-ferrous metals, powdered metal parts, sintering and brazing applications among others. Nitrogen hydrogen atmospheres have been used in brazing, and sintering and annealing applications although not necessarily at the mixture ratios of the present invention. Further, conventional control technique is to maintain the flow rate of the neutral gas somewhat constant. When the enriching or scavenger gas is added to the neutral gas there is an increase in the overall flow rate of the atmosphere within the furnace.
2.) Gas Research Institute Work
This invention was developed under a project sponsored by the assignee, Gas Research Institute, and undertaken by Surface Combustion, Inc. Gas Research Report 88/0159 entitled "High Temperature Indirect-Fired Furnace Development-Phase I", authored in part by the inventor, discusses the concepts utilized in a soft or rough vacuum furnace. Reference should be had to that report for a more detailed discussion of the various heat treat processes which are theoretically capable of being performed in a soft vacuum furnace.
The Gas Research report generally verifies that within the soft vacuum ranges defined herein and at high temperatures specified herein it is technically feasible to perform non case hardening heat treat processes with a controlled atmosphere of nitrogen along with a blend of 1 to 5% hydrogen. Heat treat processes discussed in the report included tool steel hardening, annealing stainless steel, brazing, solution annealing and sintering powdered metal parts. With the exception of annealing stainless steel, the control arrangement set forth herein is applicable to the heat treat processes identified in the GRI report as well as other processes.
The GRI concept report discussed possibly operating the furnace at partial pressure and simultaneous purging with a flowing atmosphere or alternately pumping and purging. A suggested cycle included the step of continuously supplying process atmosphere (N.sub.2 with 5% H.sub.2) to the hot zone at a rate of 50 SCFH. Control was suggested by a Residual Gas Analyzer utilizing mass spectrometry to indicate levels of oxygen and water vapor or other contaminants in the furnace with the signal being used for alarm, control/sensor etc.
3.) Measuring Instruments:
There are numerous instruments for measuring gas compositions. The Residual Gas Analyzer discussed in the GRI report is not practical for commercial application as the furnace control because of cost and time to complete the analysis. This applies to all other mass spectrometry type measuring instruments. Dew point analyzers directly measure the water vapor which is one of the principal elements to be controlled. However, the temperature at which the furnace operates and the temperature at which the sample is taken results in condensation making it difficult to correlate, for the processes under discussion, the dew point of the gas within the furnace. CO.sub.2 analyzers are not applicable because, for the processes under discussion, there is no carbon in the atmosphere. Thus by the process of elimination, oxygen probes are left.
Oxygen probes are, perhaps, the most recently developed instrument in the heat treat field. Generally the probe consists of two platinum electrodes separated by a solid electroyte usually in the form of a gas tight zirconia tube. Ambient air at the inside of the tube is in contact with one electrode while the furnace atmosphere is in contact with the other electrode vis-a-vis the outside of the tube. The differences in oxygen content induces an EMF which is correlated by well known equations, i.e. Nernst equation, to determine the partial pressure of oxygen present in the furnace atmosphere. The relationships hold fairly consistent up to temperatures of about 1900.degree. F.
Oxygen probes are universally used in the heat treat field for two applications. First, the oxygen probe is widely used to control atmosphere carburizing. The probe application is both in situ and ex situ. See "Control of Carburizing Furnace Atmospheres Using Oxygen Potential Measurements", Metallurgia and Metal Forming, December, 1972; January, 1973 pages 413-416; pages 19-22. Oxygen probes are also widely used to control external, endothermic gas generators. Atmosphere carburizing typically takes place between 1500.degree.-1800.degree. F. which is well within the temperature range of such probes. Ex situ gas generator applications are at substantially lower temperatures and at least one ex situ oxygen probe application uses a heater surrounding the probe to raise probe temperature to improve the probe readings. See Mendenhall U.S. Pat. No. 4,606,807. In endothermic gas generators it is important to drop the temperature of the endothermic gas to below about 400.degree. F. so that carbon will not form as the endothermic gas is transported from the generator to the furnace. When the gas sample is taken, ex situ, the probe readings are not consistent at the lower temperatures. Thus it is known to increase the temperature around the probe so that consistent oxygen probe readings can be taken.
While it is not possible to state with any degree of certainty what has or has not been done in the heat treat area, it is not believed that oxygen probes have been successfully used heretofore in vacuum applications for the heat treat processes under discussion. There are several reasons for this belief. First, from the discussion above, the vacuum vessel is simply pumped down. Thus there would not be any reason to sense atmosphere. Second, in those instances where the vacuum is lessened, the gas supplied is inert. Outside of such reasons, the oxygen probe, because of the pressure differential between the electrolyte tube can and may leak oxygen especially at the finer vacuum levels. Conceptually, it is not known whether this is or is not a function of electrolyte tube design. For soft vacuum application, one commercially available oxygen probe has been found by testing done by Linde not to deleteriously leak oxygen. Secondly, and more significantly, for in situ applications, the temperatures at which the furnace employing the invention is operated at will effect the electrodes rendering the probe useless.